This invention relates to time-temperature indication and, more particularly, to the development of a time-temperature indication by electrical means.
The shelf life of many products, including food, drugs, and film (photographic and x-ray), is determined by the ambient temperature to which the product is exposed. Generally, the higher the temperature, the lower the shelf life. Time-temperature indication by chemical means is known. An indicator material is separated from a reactant material for the indicator by a diffusion barrier. Passage of the reactant through the diffusion barrier depends upon the ambient temperature. In one such time-temperature indicator, disclosed in U.S. Pat. No. 3,967,579, the reactant has a clearly defined melting point at a preselected temperature; when the ambient temperature rises above the preselected value, the reactant melts and begins to flow through the diffusion barrier to the indicator. In another such time-temperature indicator, which is disclosed in U.S. Pat. No. 4,057,029, the diffusion barrier changes from an impermeable state to a permeable state at a clearly defined preselected temperature. In both the described devices, the rate of passage of the reactant above the preselected temperature is essentially independent of the ambient temperature, i.e., below the preselected temperature there is no passage of reactant, and above the preselected temperature there is passage at an almost constant rate. But, the shelf life of many products is a continuous, usually, an inverse exponential, function of ambient temperature--as the ambient temperature rises, the shelf life gradually decreases. Thus, the described time-temperature indicators do not adequately account for how much higher the ambient temperature is than the preselected temperature.
Another time-temperature indicator is disclosed in U.S. Pat. No. 2,782,749, and is based on the use of temperature sensitive materials having non-specific melting points, such as fats and waxes, which gradually become less viscous as temperature increases. These temperature sensitive materials are formed as a layer on a porous paper barrier and at or above a predetermined temperature, the fat or wax begins to diffuse through the paper barrier and eventually reaches a point of visibility on the opposite side of the barrier. The rate of diffusion of the material through the barrier is directly related to the temperature. Devices of this type, due to the nature of fats and waxes, have a relatively narrow temperature range at which they can be used and it is difficult to control the diffusion rate through the paper barrier. In addition, the paper barrier must be relatively thick, on the order of 0.75 inches, so that the device is relatively bulky, thus giving rise to handling and storage problems when attached to packages or containers.